In general, the production of the plated steel sheet is carried out by subjecting a raw material for the plated steel sheet to hot rolling, removing an iron oxide layer covering the surface of the steel sheet in a pickling equipment, subjecting to cold rolling, if necessary, and then subjecting to plating in a continuous hot dipping apparatus, an electric plating apparatus or the like. The reason why the removal of the iron oxide layer is essential in this method is due to the fact that the iron oxide layer obstructs the plating and results in a start point of peeling a plated layer to degrade the adhesion property of the plated layer.
On the other hand, JP-A=6-279967 proposes that reduction treatment is carried out in a reducing gas atmosphere without removing the iron oxide layer and thereafter hot dip galvanizing is conducted in the production of hot dip galvanized hot rolled steel sheets. Concretely, the reduction treatment is carried out in a H.sub.2 atmosphere having a high concentration of 75%.
According to the above method, the hot dip galvanizing not forming non-plated portion is realized by sufficiently conducting the reduction in a heating furnace of a continuous hot dipping apparatus without removing the iron oxide layer. However, since the H.sub.2 concentration as a reducing atmosphere is high, the pickling cost is reduced, while the cost required for the heating furnace of the continuous hot dipping apparatus largely increases.
If it is intended to plate a cold rolled raw material not requiring the reduction of the iron oxide layer in the same continuous hot dipping apparatus as mentioned above, it is necessary to change the H.sub.2 concentration to about not more than 10% because if the iron oxide layer is not existent, hydrogen is absorbed in the inside of the steel sheet during the heating and then discharged from the steel when the steel sheet becomes low temperature after the plating and hence it is vaporized at an interface to the plated layer to cause local peeling of the plated layer. Therefore, the change of the H.sub.2 concentration brings about the lowering of the productivity and the increase of the cost.